This invention relates to passivating the surface of silicon.
It is well known that the termination of a semiconductor lattice at the surface of a wafer results in an electronic configuration at the surface drastically different from that found within the bulk of the semiconductor. Situations such as distorted bonds or unsatisfied dangling bonds at the surface create electronic states within the semiconductor's forbidden energy gap. Such electronic states are localized and therefore can trap and immobilize charge and thereby degrade the performance of the device. For example: in field effect transistors (FET's) the storage of charge in surface states increases the gate capacitance, adversely affecting the high frequency response of such devices. Since such stored charge is immobile it does not contribute to current flow in the FET channel reducing the device transconductance. In photovoltaic devices, such surface states act as recombination centers for photogenerated electrons and holes reducing the output current and voltage delivered by such devices and thereby limiting their energy conversion efficiencies.
In conventional semiconductor device technology, crystalline silicon remains the dominant material out of which active devices are fabricated. A crucial property of silicon which has to a great measure been responsible for its technological importance is the existence of its oxide (SiO.sub.2) which when carefully grown on the surface of a water forms an excellent surface passivation layer nearly eliminating all surface states at the silicon--SiO.sub.2 interface. Surface state densities may be as low as 10.sup.10 /cm.sup.2 (approximately 1/10.sup.5 surface atoms). The preparation of such well passivated oxidized silicon surfaces is difficult. It requires the proper preoxidation cleaning and etching followed by a high temperature (greater than 1000.degree. C.) oxidation step, see e.g. M. M. Atalla, E. Tannenbaum, and E. J. Scheibner, Bell. Sys. Tech. J., 749 (May, 1959). As well as being a costly energy consuming process high temperature oxidation of silicon is susceptible to the degradation of the silicon wafer due to the mobility of dopants and impurities at such elevated temperatures.
Non-oxidative surface treatments of silicon have been attempted involving treatment with a variety of etchants, see, e.g., T. M. Buck and F. S. McKim, J. Electrochem. Soc. 105, 709 (1958). However, such passivations were found to be inferior to thermally grown oxides and subject to degradation in room atmosphere.
Accordingly, it would be highly desirable to overcome the problems associated with the high temperature oxidation process and produce a passivated silicon surface whose surface state density was as low as that for thermally oxidized surfaces. Such a process should be a simple low temperature process and produce a passivated surface stable under atmospheric conditions.